[Distillation FAQ]

[Gas Chromatography FAQ]

Q:  Do we distill the same mixture twice?

A:  No, use a fresh 50 mL portion each time (for simple and for fractional distillation).  Discard in the proper waste bottle.

Q:  What are the most common mistakes in the distillations?

A:  There is a lot of variety in things that might be done wrong here. That's why you should have your instructor check the setup. Placing the thermometer too high is a critical mistake because we're using boiling point in this experiment to identify the alcohols from among a list of candidate structures.  Be sure things are securely clamped, and that the joints are well greased and seated properly. See also Distillation FAQ.

Q:  Could my alcohols be any on the candidate list?

A:  Keep in mind that the unknown alcohols are saturated, acyclic (and liquid) compounds only.

Q:  How do I know when to take the aliquots during fractional distillation?

A:  That's why we do simple distillation first - you know at about what temperature the lower and higher boiling fractions will distill and you have a rough idea of the volume at which the temperature rise will be sharp. So, with that in mind, watch for the beginning of a sharp temperature rise to take the low boiling aliquot and for the point the temperature rise becomes less rapid (starts to plateau) to take the high boiling aliquot.  It is worth emphasizing that you should actually plot the simple distillation data before coming to lab to do fractional distillation.  Having said that, don't worry too much over the aliquots - the whole point is to see qualitatively what is going on during the course of distillation and interpreting this, perhaps in terms of Raoult's law.

Q:  When should I calibrate the thermometer?

A:  Since barometric pressure fluctuates, do this on the same day you perform the fractional distillation.  Simply place the thermometer in boiling water (remember to use boiling chips) to measure the correction to be applied.  Plot the corrected temperature data.

Q:  What do I do if most of the solvent in my aliquots has evaporated upon storage?

A:  Replenish with anhydrous ethyl ether to make up for lost solvent before doing GC analysis - we do not want the GC samples to be too concentrated.

Q:  Which of my GC samples should I inject first?

A:  The original (undistilled) mixture because you know it contains both alcohols, and will then know what to expect from the other injections.

©2001,2002 Daniel A. Straus